In conventional computerized tomography for both medical and industrial application, an x-ray fan beam source and a linear array detector are employed to achieve a two-dimensional (2D) image. With such an arrangement, only a single slice of an object is imaged at a time. When a 3D image is required, a “stack of slices” approach is employed, which can be inherently tedious and time-consuming.
Another approach, based on “cone beam geometry”, employs a cone beam x-ray source instead of an x-ray fan beam source and a two-dimensional array detector instead of a linear array detector. At an instant, the entire object is irradiated by a cone beam x-ray source, and therefore cone beam scanning is faster than slice-by-slice scanning using a fan beam or a parallel beam. To achieve cone beam projection data, an object is scanned, preferably over a 360 degree angular range, either by moving the x-ray source in an appropriate scanning trajectory, for example, a circular trajectory around the object while keeping the 2D array detector fixed with reference to the source, or by rotating the object while the source and detector remain stationary. In either arrangement, it is the relative movement between the source and the object which effects scanning. Image reconstruction is then accomplished in accordance with known techniques.
Direct reconstruction of x-ray attenuation coefficients or “densities” on a 3D mesh of points from 2D x-ray projection data is the basis of modem medical tomographic imaging. Currently, there are a number of commercially available X-ray systems that generate 3D images by means of computed tomography (CT). Typically in these systems, the patient is placed on an examination table, which is transparent to x-rays, and is positioned in the center of a large cylinder-shaped device such that the axis of the patient is aligned with the axis of the cylinder. Within the walls of the cylinder, a source-detector pair, formed by an x-ray point source and a linear detector array, is positioned such that the point source and the center of the detector array are diametrically opposed. The x-ray point source emits a fan-beam radiation pattern in a plane perpendicular to the axis of the cylinder. The detectors in the array are arranged linearly along the arc of a circle whose center is the point source. The detector array is positioned in the plane of the fan-beam and is wide enough to intercept the extreme rays of the fan. In the simplest arrangement, 2D projection data are obtained in the following way. The patient is exposed with the source-detector pair in an initial orientation to obtain an equiangular fan-beam projection. The source-detector pair is then rigidly rotated by an incremental angle about the cylinder axis and another equiangular fan projection is taken. The process is repeated until projection images have been obtained over a full 360°. Then, either the patient or the source-detector pair is advanced an incremental distance along the axis of the cylinder, and another series of fan projections over 360° are obtained. This process is repeated until the desired 2D x-ray projection data is accumulated. Reconstruction of a 2D density distribution within a slice through the patient can be computed from each series of fan-beam projections over 360° (three hundred sixty degrees). Finally, a complete 3D representation of the x-ray density function, ρ(x,y,z), is obtained by combining the individual 2D distributions.
There are several disadvantages to CT systems such as the one described above. For example, the accumulation of 2D projection images is a relatively slow process resulting in prolonged exposure of the patient to X-ray radiation. This results directly from the 360° fan-beam scans, which must be repeated for different positions along the cylinder axis. This is not a desirable situation since, for a given detector sensitivity, the total dosage received by the patient is increased in proportion to the number of cross-sectional scans. Another disadvantage is that such CT systems tend to be very expensive due largely to the cost of manufacturing mechanical assemblies that permit the rapid and precise translation of the source-detector pair relative to the patient. Further, such CT systems tend to be somewhat inflexible; that is, they are typically designed for specific modalities (e.g., chest, stomach, and full body x-rays).
Reconstruction of 3D density distributions from cone-beam projection images represents an alternative method for CT that overcomes many of the disadvantages disclosed above. An x-ray point source emits radiation in a cone, which is intercepted by a 2D detector array. The source-detector pair rotates rigidly about a vertical z-axis, which is aligned to the axis of the patient. Cone-beam projection images are then taken for each discrete angle of rotation over the entire angular range of 360°. The line integral of the x-ray density distribution function along the path from the point source to a location in the detector plane for a particular projection angle is related to the x-ray intensity measured at the corresponding location in the detector array for the particular projection angle
3D cone beam CT systems generally enable diagnostic procedures to be performed more quickly than with other types of CT scanning. In such CT systems that utilize 2D detectors, it is possible to reconstruct spatial volume data at extremely high resolution. In some cases, the resolution may be an order of magnitude higher than the resolution obtained with other types of detectors.
However, manipulating data at this higher resolution is generally prohibitive due to the long reconstruction times and the physical size of the reconstructed volume. High resolution 2D detectors have such a large number of pixels that the amount of data collected is extremely large, which presents problems in terms of storing, retrieving, and manipulating the data in real time. U.S. Pat. No. 6,324,243 (Edic) describes a method and apparatus for use in volumetric CT scanning systems that attempts to reduce the amount of data that is used by initially performing 3D reconstruction on projection images that have been sampled at a lower resolution, and then uses all of the acquired data associated with a particular region of interest to reconstruct a high resolution image of the region of interest. The intermediate step of performing a low resolution 3D reconstruction, while clearly having an advantage over performing a high resolution 3D reconstruction of the entire object, still requires extra computation than is necessary if the region of interest can be identified within one or more of the projection images.
U.S. Pat. No. 6,009,142 (Sauer) describes a technique for reconstructing a region of interest (ROI) in an object when the cone-beam source travels in a helical pattern around the object. Sauer's technique attempts to reduce the computation required for reconstruction by only reconstructing to a region of interest. However, Sauer's technique is not amenable for reconstructing arbitrary regions of interest. Rather, Sauer's region of interest is formed by limiting the object by planes passing through the longitudinal axis of the helix. This prohibits Sauer's technique from being used to reconstruct, for example, a small region around a tumor, if no part of the region intersects the longitudinal axis of the helix.
Accordingly, a need exists for a method and apparatus for use in a 3D cone beam CT system that enables an arbitrary region of interest to be identified in one or more of the cone beam projection images, and provides for the generation of a high resolution 3D reconstruction of the particular region of interest.